Most semi-automatic (and many fully automatic) firearms eject spent casings to one side. Ejection is accomplished by cooperation between an extractor and an ejector as the spent casing is removed from the chamber when the action cycles. Generally, the extractor grips an edge of the cartridge rim and the ejector contacts the base at an approximately diametrically opposite point to fling the shell through the ejection port of the firearm's receiver. AR-pattern firearms include, but are not limited to, the AR10, AR15, M16, M4, and other variants in a rifle or pistol configuration. In an AR-pattern firearm, ejection is generally to the right, because most shooters are right-handed, although left-handed AR-pattern firearms have been made in which every part is produced as a mirror image of the standard. In an AR-pattern firearm, ejection is not directly to the side, but rather at an inclined angle, approximately 67.5 degrees from vertical (or 22.5 degrees above horizontal).
Various mechanisms have been used to allow selection between left-hand ejection and right-hand ejection in a single firearm. These, however, use complex switching mechanisms and/or many parts non-standard to an AR-pattern firearm.